The present invention relates to a hybrid switch which has semiconductor switching elements connected in parallel to a main contact of an electromagnetic contactor used for switching a motor load or the like, and which is adapted to operate the semiconductor switching elements for a short time at the time of the actuation and cutting off of the electromagnetic contactor.
The hybrid switch of this type ensures that no arc is produced between contacts of a mechanical contact, which is a main contact of an electromagnetic contactor, by causing semiconductor switching elements to perform the turning on and off of an energization current during the actuation and cutting off of the electromagnetic contactor.
FIG. 14 is a circuit diagram of a hybrid switch which illustrates a conventional device a patent application for which was filed by the present applicant (Japanese Unexamined Patent Publication No. Hei. 4-354374).
In FIG. 14, reference numeral 51 denotes an a.c. power supply; 52, a load connected to the a.c. power supply 51 via a main contact 53a of an electromagnetic contactor 53; 53, the electromagnetic contactor having the main contact 53a and an auxiliary normally-closed contact 53b; 54, a triac (a semiconductor switching element) connected in parallel to the main contact 53a of the electromagnetic contactor 53; and 55, a gate trigger resistor connected in series with the auxiliary normally-closed contact 53b of the electromagnetic contactor 53, its connection with the auxiliary normally-closed contact 53b being connected to the gate G of the triac 54.
In the hybrid switch shown in FIG. 14, when the electromagnetic contactor 53 is in a cut-off state, the triac 54 remains off since the auxiliary normally-closed contact 53b is closed and the circuit between the gate and the cathode is shorted. On application of an operation input voltage signal to the electromagnetic contactor 53, the electromagnetic contactor 53 starts operation, and an unillustrated movable iron core starts to move toward a fixed iron core, whereupon the auxiliary normally-closed contact 53 is opened before the main contact 53a is closed, thereby supplying a firing signal to the gate G of the triac 54 via the gate trigger resistor 55 and turning on the triac 54. After the triac 54 is turned on, if the main contact 53a of the electromagnetic contactor 53 is closed, most of the load current flowing across the triac 54 flows to the load 52 via the main contact 53a. Then, when there is no longer the operation input voltage signal which was applied to the electromagnetic contactor 53, the main contact 53a is opened. At this time, since the firing signal is applied to the gate G of the triac 54 via the gate trigger resistor 55, the triac 54 is turned on, and the load current flowing through the main contact 53a flows via the triac 54. When the auxiliary normally-closed contact 53b is closed after the main contact 53a of the electromagnetic contactor 53 is opened, the circuit between the gate and the cathode of the triac 54 is shorted, and the firing signal which was being supplied to the gate G is stopped, so that the triac 54 is turned off at a point of time when the load current from the a.c. power supply 51 passes through the zero point. Thus, when the electromagnetic contactor 53 is actuated and cut off, the triac 54 undergoes an on operation for a short duration, thereby preventing the occurrence of an arc resulting from the closing and opening of the main contact 53a. Since it suffices if the triac 54 undergoes the on operation for a short duration to allow the load current to flow, an element of a small capacity for conduction for a short duration is used as the triac 54.
With the conventional device shown in FIG. 14, since the auxiliary normally-closed contact 53b is open when the electromagnetic contactor 53 is on, a load current, though small, flows across the gate G of the triac 54. For this reason, if the main contact 53a of the electromagnetic contactor 53 undergoes a faulty contact, the load current is applied to the gate G to turn the triac 54 on, causing all the load current to flow across the triac 54. Since the element of a small capacity for conduction for a short duration is used as the triac 54 as described above, if the load current continues to flow due to the faulty contact of the main contact 53a, there is the risk of heat being generated, resulting in thermal breakdown. To prevent such thermal breakdown of the triac 54, it is conceivable to use a large-capacity element as the triac 54. In that case, however, it is necessary to provide a cooling member for heat dissipation in the case where the load current continues to flow across the triac 54. In addition to the fact that the large-size triac 54 causes the element itself to be expensive, since the cooling member is required, there are drawbacks in that the hybrid switch becomes expensive and large in size. In the event that the auxiliary normally-closed contact 53b undergoes a faulty contact in addition to the faulty contact of the main contact 53a, the same drawbacks as those described above are encountered since the load current is constantly supplied to the gate G of the triac 54.
In addition, when the opening and closing of the load 52 is effected at a high frequency, the hybrid switch, particularly the triac 54, undergoes the on operation frequently. For this reason, the electric power occurring in the triac 54 increases, and the amount of heat generated becomes large, so that there is the drawback in that thermal breakdown occurs in a small-capacity triac.
Furthermore, as for the electromagnetic contactor, if the voltage value of the operation input voltage signal is at a prescribed value or higher, sufficient attraction occurs between the fixed iron core and the movable iron core, and complete actuation takes place. On the other hand, if the voltage value of the operation input voltage signal is below the prescribed value, and the operation input voltage signal of a voltage value lower than the specified value is applied, sufficient attraction does not occur between the fixed iron core and the movable iron core, so that the attraction and release of the movable iron core is repeated. Consequently, the main contact 53a and the auxiliary normally-closed contact 53b of the electromagnetic contactor are repeatedly turned on and off, so that the triac 54 frequently undergoes the on operation. In this case as well, there is the drawback in that thermal breakdown occurs in a small-capacity triac.